1. Field of the Invention
The present invention relates to a code division multiplex communication apparatus for transmitting and receiving image data.
2. Description of the Related Art
In the spread-spectrum communication method using the direct spread technique, a baseband signal having a much wider bandwidth than original data is generated from a normal transmitting digital signal by the use of a series of spread codes, such as pseudo noise codes (PN codes) or the like. The resulting signal is further subjected to modulation according to Phase Shift Keying (PSK), Frequency Shift Keying (FSK), or other modulation methods, so as to be converted to a radio frequency (RF) signal. The RF signal is then transmitted. In the receiving end, a de-spread operation is performed by obtaining a correlation between the transmitting signal and a receiving signal by the use of the same spread codes used by the transmitting end. The receiving signal is then converted into a narrower band signal having a bandwidth corresponding to the original data, followed by normal data demodulation performed on the converted receiving signal. The original data is thus reproduced.
As discussed above, in the spread-spectrum communication method, the transmitting bandwidth is much wider than the information bandwidth. Thus, according to this method, when using a fixed transmitting bandwidth, the transmitting speed disadvantageously becomes much lower than by the normal narrower-bandwidth modulation method. Image transmission using such a spread-spectrum method is, in general, performed by the following procedure. That is, an image input from a camcorder is incorporated as a still image into a personal computer and is then output to a spread-spectrum transmitting section via an interface. The image signal is subjected to a spread-spectrum operation in the transmitting section and then sent to a transmission line through an antenna. In the receiving end, the signal received in an antenna is de-spread in a spread-spectrum receiving section and is incorporated as digital data into a personal computer via an interface. Subsequently, the data is converted into an image signal in the personal computer, and the image is then displayed on a monitor.
The transmittable data rate of signals by the above type of spread-spectrum image transmitting apparatus ranges at most from 1 Mbps to 2 Mbps. Therefore, the transmittable image size is restricted to, for example, only one non-compressed full-color image with a 320.times.240 dots and 24 bits, each RGB having 8 bits, and only one image of the above size can be transmitted per second (1.84 Mbps). At this rate, it is impossible to transmit, for example, even a television image with only a few fields per second. If moving pictures are required to be transmitted, even at this low rate, expensive image compression LSI circuits are further required. This increases the cost of the overall apparatus.
For overcoming the drawback of this low data-transfer rate, the code division multiplex method is available. In this method, high-speed data is converted into low-speed parallel data, and the respective bits of the converted data are spread-spectrum modulated by the use of a series of the different spread codes. The modulated bits of data are added, and the added data is further converted into an RF signal, which is then transmitted. This method enables high-speed data transmission under a condition of a fixed transmitting bandwidth without impairing the spread rate of the spread modulation.